Printed circuit boards (also referred to herein as PCBs), laminate chip carriers, and the like organic products permit formation of multiple circuits in a minimum volume or space. These structures are known to comprise a “stack” of electrically conductive layers of signal, ground and/or power planes (lines) separated from each other by a layer of organic dielectric material. Selected ones of the lines in one layer are often in electrical contact with corresponding selected ones in one or more other layers by plated holes passing through the dielectric layers. The plated holes are often referred to as “vias” if internally located, “blind vias” if extending a predetermined depth within the board from an external surface, or “plated-thru-holes” (PTHs) if extending substantially through the board's full thickness. By the term “thru-hole” as used herein is thus meant to include all three types of such board openings.
Methods for making such PCBs, chip carriers and the like typically comprise fabrication of separate inner-layer circuits (circuitized layers), which are formed by coating a photosensitive layer or film over a metal (usually copper or copper alloy) layer on a dielectric “base” layer. The organic photosensitive coating is imaged, developed and the exposed copper is etched to form the desired circuit, e.g., a pattern of conductor lines, pads, etc. After etching, the photosensitive film is stripped from the copper, leaving the circuit pattern on the surface of the base layer. This methodology is also referred to as photolithographic processing in the PCB art and further description is not deemed necessary. Following the formation of individual circuit layers (some including circuitry on opposite sides of the dielectric), a multilayer “stack” is formed by preparing a lay-up of these formed inner-layers, ground planes, power planes, etc., typically separated from each other by the mentioned dielectric, organic pre-preg material typically comprising a layer of glass cloth (usually fiberglass) impregnated with a partially cured material, one common example referred to as a “B-stage” epoxy resin. Such an organic material is also referred to in the industry as “FR-4” dielectric material. The top and bottom outer layers of the stack usually comprise copper clad, glass-filled, epoxy planar substrates with the copper cladding comprising exterior surfaces of the stack. The stack is laminated to form a monolithic structure using heat and pressure to fully cure the B-stage resin. The final stack so formed thus has the metal cladding on at least its upper surface but this cladding may also be positioned on the bottom or undersurface of the stacked structure. Exterior circuit layers are then usually formed in the copper cladding using photolithographic procedures similar to the procedures used to form the inner-layer circuits. That is, a photosensitive film is applied to the copper cladding, exposed to pattern activating radiation and developed. An etching solution such as cupric chloride is then used to remove copper bared by the development of the photosensitive film. Finally, the remaining photosensitive film is removed to provide the exterior circuit layers. Elements of such layers, e.g., conductive pads, may be used then to have electrical components mounted thereon. One such example of an electrical component is a chip carrier, or even a single semiconductor chip, both of which may be mounted on the external pads using solder balls or some other known process, e.g., wire-bonding. If the structure so formed is to serve as a chip carrier, e.g., having one or more chips mounted on one surface, the pads on the undersurface may serve as connecting pads for appropriate conductors, e.g., solder balls, which may couple the carrier and its chip(s) to a designated underlying circuitized substrate, typically a multilayered PCB.
It is known in the art to use thru-holes of the above type to electrically connect various individual circuit layers within the structure, as well as to the aforementioned outer conductive surface(s). As mentioned, such thru-holes may pass through only parts of the stacked substrate and may also be “buried” or hidden therein. Such thru-holes may be formed prior to the formation of circuits on the exterior surfaces by drilling holes through the stack at appropriate locations. For internally positioned thru-holes, such holes are usually formed within the individual circuitized layers prior to incorporation within the multi-layered structure and final lamination thereof. In both methods, the bare hole walls are usually subjected to at least one pre-treatment step after which the walls are catalyzed by contact with a plating catalyst and metallized, typically by contact with an electro-less or electrolytic copper plating solution. If the thru-holes are PTHS, interconnections are thus formed between selected ones of the circuitized layers of the multilayered final product which have one or more conductive lines or elements in contact with the inner conductive layer of the PTHS. If the thru-holes are individually formed within selected layers and then coupled to one another during product stacking, connectivity may be accomplished using a conductive paste or the like. Such pastes are known to include a conductive metal such as silver in the form of flakes. Following formation of the conductive thru-holes in multilayered structures such as PCBS in which the thru-holes are provided as PTHS, the aforementioned exterior circuits (outer-layers) are formed. Such external formation may also occur when stacking layers already having thru-holes formed therein, albeit it is possible to form the two outer conductive layers prior to stacking and lamination. When external components are mounted on the substrate and coupled to the external conductors, e.g., pads, thereon, it is thus seen that said components are then capable of being electrically coupled to other such components through the substrate's internal circuitry.
Electrical testing of such formed substrates which utilize conductive paste as the coupling medium (either alone or in combination with plating on the inner walls of formed openings) involves testing resistances of the various circuit paths across (through) the paste. Such testing is obviously very important considering the thousands of circuits often contained within complex substrates of the type being produced today. Failure of just one circuit path may in turn necessitate scrapping or costly repair of the substrate, obviously a result which manufacturers wish to avoid. On some occasions, the resistance of one or more connections may be considered relatively high, and thus unacceptable. As defined in greater detail herein-below, the present invention provides a method of improving such paste connections in the finally bonded (e.g., laminated) multilayer structure having many such internal connections therein in a facile and non-destructive manner. Such a method is considered a significant advancement in the art.
Examples of various multilayer substrates, including some which utilize conductive paste for interconnections, are described below.
In U.S. Pat. No. 6,828,514, issued Dec. 7, 2004, there is defined a multilayered PCB including two multilayered portions, one of these able to electrically connect electronic components mounted on the PCB to assure high frequency connections there-between. The PCB further includes a conventional PCB portion to reduce costs while assuring a structure having a satisfactory overall thickness for use in the PCB field. Coupling is also possible to the internal portion from these components. This patent is assigned to the same Assignee as the instant invention.
In U.S. Pat. No. 6,809,269, issued Oct. 26, 2004, there is defined a circuitized substrate assembly and method for making same wherein the assembly includes individual circuitized substrates bonded together. The substrates each include at least one opening, one of which is substantially filled with a conductive paste prior to bonding. Once bonded, the paste is also partially located within the other opening to provide an effective electrical connection therewith. One example of a product using this technology is a chip carrier. This patent is also assigned to the same Assignee as the instant invention.
In U.S. Pat. No. 6,762,496, issued Jul. 13, 2004, there is described a sintered aluminum nitride substrate which has a “via” hole and an internal electrically conductive layer with, allegedly, high thermal conductivity and high adhesion strength between the sintered aluminum nitride substrate and the internal electrically conductive layer or the via hole. The substrate consists of an internal electrically conductive layer, at least one electrically conductive via hole formed between the internal electrically conductive layer and at least one surface of the substrate, wherein the thermal conductivity of the aluminum nitride sintering product at 25 degrees Celsius (C.) is described as being 190 W/mK or more, with a corresponding adhesion strength between the aluminum nitride sintering product and the internal electrically conductive layer also mentioned.
In U.S. Pat. No. 6,641,898, issued Nov. 4, 2003, there is described a heated and pressed printed wiring board which is made by filling “via” holes formed in layers of insulating film of the wiring board with an interlayer conducting material. The insulating film is stacked with conductor patterns, and each conductor pattern closes a hole. The interlayer conducting material forms a solid conducting material in the holes after a heating and pressing operation. The solid conducting material includes two types of conducting materials, the first type being a metal and the second type including an alloy formed by the metal and conductor metal of the conductor patterns. The first type of conducting material includes indium particles, tin and silver, wherein tin accounts for approximately 20-80 weight percentage of the solid conductive material, and the second type of conducting material includes an alloy comprised of the solid conductive material and the conductor metal. The conductor patterns are electrically connected reliably without relying on mere mechanical contact.
In U.S. Pat. No. 6,593,534, issued Jul. 15, 2003, there is described a method for producing a multilayer printed or wiring circuit board, and more particularly a method producing so-called z-axis or multilayer electrical interconnections in a wiring structure in order to be able to provide for an increase in the number of inputs and outputs (I/O) in comparison with a standard printed wiring board (PWB, this being an alternative term for a PCB) arrangement, and a printed wiring board produced by the method. The thru-holes of the structure are filled with conductive paste.
In U.S. Pat. No. 6,479,093, issued Nov. 12, 2002, there is described a laminate circuit structure assembly that comprises at least two modularized circuitized plane subassemblies and a joining layer located between each of the subassemblies wherein the subassemblies and joining layer are bonded together with a cured dielectric. The subassemblies and joining layer are electrically interconnected with bondable electrically conductive material. The joining layer comprises dielectric layers disposed about an internal electrically conductive layer. The electrically conductive layer has a via and the dielectric layers each have a via of smaller diameter than the vias in the electrically conductive layer and are aligned with the vias in the electrically conductive layer. The vias are filled with electrically bondable electrically conductive material for providing electrical contact between the subassemblies.
U.S. Pat. No. 6,388,204, issued May 14, 2002, is the parent of U.S. Pat. No. 6,479,093, and describes similar structures and processes.
In Published Patent Application 2002/0050586, issued May 2, 2002, there is described an electro-conductive paste for use in making ceramic substrates containing from about 5 to 18 percent by weight of an organic vehicle consisting of a solvent and a binder, from about 80 to 93 percent by weight of an electro-conductive metal powder in a spherical or granular shape and with a particle diameter in the range of about 0.1 to 50 microns, and from about 2 to 10 percent by weight of a resin powder with a particle diameter in the range of about 0.1 to 50 microns which is insoluble in the solvent and has a low level of water absorption. This paste may be used for forming via hole conductors to be converted to external electrode terminals for the resulting ceramic products.
In U.S. Pat. No. 6,143,116, issued Nov. 7, 2000, there is described a multilayer wiring board formed by laminating a plurality of circuit board “units” each including an insulating board containing at least a thermosetting resin, and a wiring circuit layer formed on the surface of the insulating board, wherein the board is provided with via hole conducting passages so as to electrically connect the wiring circuit layers of the neighboring circuit board units. The via hole conducting passages are filled with a conducting paste, the wiring circuit layer buried in the surface of the board in a manner that the possesses a flat surface for mounting flip chips. This patent mentions in column 11 that resistance of the conducting passages can be further decreased through application of a pulse current to the via hole connections, resulting in a “welding” of the metal particles of the conductive paste.
In U.S. Pat. No. 6,120,708, issued Sep. 19, 2000, there is described a conductive paste for forming via-holes in a ceramic substrate, which paste contains about 80-94 weight percentage spherical or granular conductive metal powder having a particle size of about 0.1-50 microns, 1-10 weight percentage resin powder which swells in a solvent contained in the conductive paste and has a particle size of about 0.1-40 microns, and about 5-19 weight percentage of an organic vehicle. The paste is described as hardly generating cracks during firing to thereby attain excellent reliability in electric conduction and which can provide a via-hole or through hole having excellent solderability and platability in a ceramic substrate structure.
In U.S. Pat. No. 5,956,843, issued Sep. 28, 1999, there is described a multilayer printed wiring board and method of making thereof, in which a substrate constitutes a plurality of dielectric layers having electrically connecting components formed on the surfaces thereof so as to be either arranged externally or internally intermediate adjacent layers, and which are provided with vertically aligned through-holes or vias communicating between adjacent layers. Each of the holes may be equipped with electrically conductive plated layers or similar structures covering the surfaces of the holes or vias, and a conductive or non-conductive material, such as in the form of a paste, may be filled into each plated hole.
In U.S. Pat. No. 5,891,283, issued Apr. 6, 1999, there is described a conductive paste for use in forming ceramic substrates in which the composition consists of an organic vehicle, copper powder and an organo-metallic resinate which includes, as the metal, at least one metal selected from the group consisting of Pt, Ni and Bi. The amount of the metal component in the organo-metallic resinate is in the range of about 0.1 to 5 weight percentage with respect to the total amount of the copper power and the metal component. The copper powder has preferably an average diameter in the range of about 2 to 30 microns.
In U.S. Pat. No. 5,817,404, issued Oct. 6, 1998, there is described a printed circuit board in which electrode layers can electrically be connected by an inner-through-hole connection. A cover film is laminated on both surfaces of a resin impregnated fabric sheet, holes are formed by laser beams in the direction of the thickness of the sheet and cover film, and an electrically conductive paste that contains the electrically conductive particles and a thermosetting resin is filled in the holes. The cover film is removed, a copper foil is placed on both sides of the sheet, and pressed and heated, and the resin component of the sheet and that of the electrically conductive paste are hardened. The copper foil is etched to form circuit patterns, and further etching removes the electrically conductive particles present at a surface layer portion so that an electrical insulating layer is thus formed.
The relative complexity of the above organic products (those including organic dielectric layers, including the aforementioned PCBs and laminate chip carriers) has increased significantly over the past few years, especially as such products increase in demand over those of the earlier ceramic variety. For example, PCBs for mainframe computers may have as many as thirty-six layers of circuitry or more, with the complete stack having a thickness of as much as about 0.250 inch (250 mils). Laminate chip carriers, in turn, may have as many as fifteen circuit layers as part thereof. Such organic products are known with three or five mil (a mil being one thousandth of an inch) wide signal lines and twelve mil diameter thru-holes, but for increased circuit densification in many of today's products, the industry is attempting to reduce signal lines to a width of two mils or less and thru-hole diameters to two mils or less. Such high densification understandably mandates the most efficient means of interconnecting the respective layers in the smallest space available and using the best materials possible. As defined herein, the present invention is able to assure that such connections of the conductive paste type will possess the proper resistance levels and thus meet the stringent requirements demanded of today's substrates.
As stated, it is believed that a method of improving the conductive paste connections in formed circuitized substrates in a facile manner such that these connections will each possess an optimum, desired resistance level for such substrates, including those having highly dense circuitry, will constitute a significant advancement in the art.